1. Field of the Invention
This invention relates to a capped modular microwave integrated circuit (MMIC) and a method of fabrication thereof.
2. Brief Description of the Prior Art
Gallium arsenide MMICs are conventionally designed for use in an open cavity and with a perimeter wire bond interface. This type of interface limits microwave performance because of the parasitic effects of the wire interface and the coupling effects of the open cavity. Thus, the currently used MMIC crossover techniques have an effect on device performance. Packaging metrics for size and weight are limited because of the space required for the wires and the cavity. Also, the design of the MMIC to create an edge interface requires increased MMIC size.
In general, prior art gallium arsenide processing for high frequency (about 10 GHz) applications started with a wafer having a thickness of about 25 mils with normal processing taking place on a surface thereof. The wafer is then ground back, generally to a thickness of about four mils. Backside processing then takes place including the etching of vias and metallization of the backside of the wafer. The wafer is then diced to provide the finished parts. A problem with these parts is that they now have a thickness of about 4 mils, it being known that gallium arsenide is not a strong material and is a poor thermal conductor. It is therefore easy to chip or break the parts during further handling and/or processing and there is also a thermal penalty imposed due to the thickness of gallium arsenide. This also impedes the use of even thinner gallium arsenide chips and the benefits that can be derived therefrom.
One prior art approach known as high density interconnect (HDI) eliminates bond wires. This concept uses MMICs that are mounted face up in a pocketed base to create a planar surface. A multilayer circuit is manufactured with direct interface to the MMICs. The circuit is ablated in the area of the circuit over the active zones of the MMIC. The packaging is completed with an enclosure to create a cavity. This concept achieves improved MMIC interface performance. Coupling within the cavity as well as size and weight are also reduced. MMIC redesign is not required for this approach. However, this concept is still in an open cavity and therefore has electromagnetic fields disposed above the open cavity.
A further prior art approach involves the use flip chip technology to eliminate bond wires. This concept uses MMICs that are mounted upside down on a multilayer circuit, such as low temperature ceramic cofire (LTCC). The MMIC is redesigned with a coplanar waveguide topology to accommodate the effects of the adjacent substrate. The packaging is completed with an enclosure to create a hermetic cavity. This concept has thermal limitations. The redesigned MMICs are not restricted to perimeter interface and can be reduced in size and have improved power and microwave interfaces. Coupling within the cavity as well as size and weight are reduced.